At present there are two basic types of ophthalmoscopes; the direct ophthalmoscope and the indirect ophthalmoscope. Both types have advantages in certain circumstances and disadvantages in other circumstances.
In the direct ophthalmoscope a light source is used to directly illuminate a patient's eye and the optical system of the patient's eye functions as a simple magnifier, thereby allowing the observer to see an erect, magnified image of the patient's fundus. The direct ophthalmoscope provides a means of illuminating the fundus and of observing the illuminated area. The illumination is usually provided by means of a light source and a reflecting prism or mirror whereby the beam of light can be conveniently directed through the pupil. It will be appreciated that the illuminated fundus is viewed under conditions analogous to a magnifying glass. The retina is at the focus of the dioptric system of the eye and consequently an erect, magnified, clearly focussed image of the fundus is observed. The magnification is about fifteen times depending on the condition and characteristics of the observer's and patient's eyes, but the field of view is quite small, only about 61/2or thereabouts. The actual area of the fundus in view at any time also depends upon the distance of the observer from the pupil of the patient's eye, and to a small extent on the patient's pupil diameter. Thus, while the direct ophthalmoscope provides an erect and relatively highly magnified image, the actual area of the eye being viewed is quite small and an extensive scan of the entire surface of the eye is required in order to gain a complete picture of the eye at this magnification.
In the indirect ophthalmoscope, which can be either binocular or monocular, a condensing lens positioned in front of the patient's eye produces an inverted image of the patient's retina. This image can then be viewed by the observer either with or without the aid of additional lens systems. With most commercial instruments, the condensing lens is hand-held and the lens system is mounted on a head band, although a hand-held device and a spectacle configuration are available. In most cases, the observed image is inverted, Ln some it is not, The retinal image is viewed with a magnification in the range 2.5 to 5 times and a field of view generally around 35.degree. to 65.degree., depending on the precise optical configuration. Thus, the indirect ophthalmoscope, when provided with a suitable relay lens system, provides an inverted or erect image which is only moderately magnified but entails a relatively extensive field of view compared with direct ophthalmoscopes.
From the foregoing it is clear that there is a compromise in the use of both types of ophthalmoscopes. Whereas the direct instrument produces a greatly magnified image, it has a very small field of view and the indirect instrument has a large field of view but produces an image which is only slightly magnified. Accordingly, both instruments must be used in conjunction with each other for a full comprehensive study of a patient's eye when attempting to detect an abnormal condition. One problem with using two instruments is that when an area of interest with respect to an abnormal condition is located, say using the indirect ophthalmoscope because of its large field of view, the magnification is not great enough to investigate fully the precise nature of the abnormality. However, when an observer changes to the direct ophthalmoscope in order to view the abnormality under increased magnification, the precise location of the abnormality cannot be easily determined and a scan of the eye must be undertaken to locate the exact position of the abnormality. Therefore, an operator is continually confronted by a compromise situation as to which type of instrument to use and many investigations require the successive or alternate use of the two different instruments.
U.S. Pat. Nos. 4502766 and 4666268 disclose fundus cameras incorporating a zoom lens system for varying magnification and field of view. This concept is, however, not readily applicable to ophthalmoscopes With the arrangement disclosed in these patents, the observer will need to adjust the position of his eye as the zoom lens system is adjusted. This is not a serious objection for an instrument having the primary function of recording a photographic image, but would render an ophthalmoscope less than satisfactory. A linear optical configuration of the kind proposed for the fundus cameras of the aforementioned US patents would also render an ophthalmoscope somewhat cumbersome in use.